The invention relates to a method for recording audio information on a record carrier, the method comprising the steps of encoding the audio information to compressed audio data having a variable bitrate, recording the compressed audio data at a transfer speed, and controlling the transfer speed in dependence on the variable bitrate.
The invention further relates to a device for recording audio information on a record carrier, the device comprising compression means for encoding the audio information to compressed audio data having a variable bitrate, recording means for recording the compressed audio data at a transfer speed, and a control unit for controlling the transfer speed in dependence on the variable bitrate.
The invention further relates to a record carrier carrying audio information, which audio information is encoded to compressed audio data having a variable bitrate, and transfer speed information for controlling a transfer speed at playback.
The invention further relates to a playback device for retrieving audio information from the record carrier, the device comprising reading means for retrieving the compressed audio data at a transfer speed, and a control unit for controlling the transfer speed.
Such a method and apparatus are known from Research Disclosure 36411, document D1 in the list of related documents. The document discloses a system for variable speed recording of audio information on a record carrier, in particular speech on a magnetic tape. Compression of the audio information is used to minimize tape consumption and realize a high effective audio information density, i.e. the amount of audio information per unit of record carrier surface. When data is recorded on the tape at a constant spatial data density the data transfer speed is proportional to the tape transport speed. The known recording apparatus comprises an audio compressor for encoding the audio information to digital audio data having a variable bitrate. A channel encoder formats the compressed audio data and passes the data to a recording head via a smoothing buffer. The actual bitrate of the compressed audio data at the output of the audio compressor is measured by a bitrate determining means, which means controls the tape transport speed in dependence on the actual bitrate. The buffer is kept half full during normal operation and will absorb sudden changes in the actual bitrate, while the tape transport speed can be adapted according to the data rate changes at the output of the smoothing buffer. A speed control signal, indicating the actual tape transport speed at the moment of recording, may be recorded on the record carrier, e.g. a clock signal in a separate track of the tape. An optical disc is mentioned as record carrier in a further embodiment, which embodiment comprises a buffer sufficiently large to store the data recordable in one winding of the track. Recording is performed for one complete revolution of the disk and then halted until the buffer again contains sufficient data. The average data transfer speed of the disc is controlled by jumping back, i.e. moving the head to the previous track when recording is halted, while the transport speed of the disc relative to the recording head, called the linear speed, is not changed. Those skilled in the art are directed to the following related documents:
(D1) Research Disclosure number 36411. August 1994, page 412-413 PA1 (D2) PCT/IB97/01156 (PHN 16.452) 1 bit ADC and lossless compression of audio PA1 (D3) PCT/IB97/01303 (PHN 16.405) Audio compressor PA1 (D4) EP-A 402,973 (PHN 13.241) Audio compression PA1 (D5) 'A digital decimating filter for analog-to-digital conversion of hi-fi audio signals', by J. J. van der Kam in Philips Tech. Rev. 42, no. 6/7, April 1986, pp. 230-8 PA1 (D6) 'A higher order topology for interpolative modulators for oversampling A/D converters', by Kirk C. H. Chao et al in IEEE Trans. on Circuits and Systems, Vol 37, no. 3, March 1990, pp. 309-18.
The above citations are hereby incorporated in whole by reference.